In radiation imaging extensive use has been made of lead collimators. These are essentially grid-like screens typically made of lead configured to have apertures which only permit transmission of parallel or near parallel rays to a detector or imaging means, typically a gamma camera. Lead collimators generally suffer from low resolution and attempts to increase resolution result in lowered efficiency. For this reason, coded aperture masks are being used to replace lead collimators.
Coded aperture masks consist of a pattern of apertures in a material that has a high attenuation coefficient for the type of radiation being imaged. The array of apertures is arranged in a material such as tungsten when used in the imaging of gamma-rays. Typically, the tungsten is 1-2 mm in thickness, with, for example, 88 000 apertures arranged in a pre-determined manner, for imaging gamma-rays from a source such as a human body, as used in diagnostic nuclear medicine.
Coded aperture masks may be used as an alternative to various types of collimators in gamma-ray imaging. The apertures have the potential to increase the signal-to-noise ratio (SNR) of the system [1], and can theoretically be applied advantageously to diagnostic imaging in nuclear medicine. The increased SNR can be manipulated to improve image resolution, to shorten imaging time, or to reduce the patient's dose of radioactivity.
Coded aperture masks have been used extensively in astrophysics, where far-field imaging conditions hold. Such conditions allow for the acquisition of images that are close to perfect for two-dimensional (2D) noise-free data [2]. The near-field conditions of nuclear medicine, however, cause the image to be corrupted by near-field artifacts.
Past research has indicated apertures that are optimal for the purposes of nuclear medicine [3]. Although a reduction of near-field artifacts can be achieved by taking a second image with a rotated aperture, and by then summing the two sets of data according to the method of Accorsi [4], ghosting of the object becomes prominent when imaging over a wide field of view. This approach is further described in WO 20021056055.
Coded aperture imaging requires that for each point of the source, the aperture pattern must be projected onto the detector. This results in overlapping aperture patterns, each shifted and weighted according to the location and the intensity of the specific point source that projected the pattern [5].
Theoretically, this acquisition process is modeled by convolving the source with the aperture pattern. The image is reconstructed by correlating the encoded data with the original coded aperture pattern [5]. This pattern is designed such that a unique reconstruction exists.
Convolution implies that a point source must be imaged equally by each pinhole of the coded aperture, without change in intensity, and with the image of the point source falling directly below the pinhole. The decoding procedure performs correctly under these theoretical conditions, but in practice the convolution model does not hold and near-field artifacts remain a problem.
Whilst attempts at reducing near-field artifacts, such as that described in WO 2002/056055, do show promise, when imaging over a wide field of view the images that are obtained still contain such near-field artifacts.